The present invention relates to a method of manufacturing disk brake pads for brakes of various vehicles including automobiles and other industrial machines, and highly reliable brake pads manufactured by the method.
Generally, disk brake pads are formed to a metal back plate by bonding a friction member (liner) to a metal back plate. The friction member is formed by binding together reinforcing fibers such as inorganic fibers including mineral fibers, steel fibers, glass fibers, ceramic fibers or their composites, or composites of such inorganic fibers and organic fibers such as aramid fibers, friction adjusting agents such as graphite and barium sulfate, and fillers by means of a thermosetting binder such as a phenolic resin.
Not only do such pads have to be high in friction coefficient and wear resistance of their friction members, but also the bond strength between the friction member and the back plate has to be maintained stably so that the friction member will not peel off the back plate.
In order to prevent cracks that may develop at the interface between the friction member and the back plate due to differences in thermal expansion resulting from braking heat, it is an ordinary practice to interpose a bonding layer having a heat insulating and a damping function between the friction member and the back plate. Since back plates are formed from a ferrous material, it is necessary to take suitable measures to prevent rusting on the joint area between the back plate and the friction member. Otherwise, the bond strength between these members would lower as rusting progresses.
Rusting on the joint surface begins to progress when the friction member of the pad contains water with prolonged use and the water content reaches the back through pores in the friction member. If the porosity in the friction member is sufficiently low, rusting factors such as water cannot reach the interface, so that rusting can be avoided. Another conventional means to prevent rusting is to subject the back plate to dacrotizing treatment. But this is rather expensive. Thus, in order to prevent rusting, it is more advantageous to reduce the porosity to prevent water from permeating into the friction member.
But it is necessary to keep the porosity above a predetermined level at the frictional contact surface of the friction member. Otherwise, its frictional surface would be heated up excessively. This leads to increased production of decomposition gases such as organic gases and a dramatic reduction in its friction coefficient. In order to avoid this, non-asbestos pads should have a porosity of about 15%, which is too high to effectively prevent the water content from reaching the interface.
We, the present inventors, examined friction members having multi-layered structures comprising a frictional contact portion having a porosity higher than a predetermined level, and a bonding layer adjacent to the back plate and having a lower porosity. A friction member having such a multi-layered structure, i.e. a friction member whose porosity changes in the direction of thickness, is disclosed in Unexamined Japanese Patent Publication 56-167929.
The pad disclosed in the above publication is manufactured by pre-forming a tablet having a higher density and a tablet having a lower density, superimposing them, and heating them. This method requires a facility for pre-forming and is low in productivity and high in production cost.
Such pre-formed tablets are flat at their joint surfaces, so that their reinforcing materials cannot show sufficient reinforcing effect at their interfaces. The pad thus formed tends to be low in bond strength between layers and thus low in resistance to separation between layers.
An object of the present invention is to provide a method of manufacturing disk brake pads which effectively solves these problems, and to provide pads formed by this method which are less likely to suffer separation between layers.
Another object of the present invention is to provide a fiber-containing pelletized material used for the production of disk brake pads and its manufacturing method.